The present invention relates generally to gas turbine engines and, more particularly, is concerned with a thermally-tuned rotary labyrinth seal for a gas turbine engine.
2. Description of the Prior Art
Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected for driving a rotor which, in turn, drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbofan engines, and turboprop engines.
A recent improvement over the turbofan and turboprop engines is an unducted fan engine such as disclosed in the first U.S. patent application cross-referenced above. In the unducted fan engine, the power turbine includes counterrotating rotors with turbine blades defining counterrotating airfoil stages which drive in corresponding fashion unducted fan blades radially located with respect to the power turbine. The fan blades of the unducted fan engine are variable pitched blades to achieve optimum performance. During operation, fuel efficiency of the engine can be increased by varying the pitch of the blade to correspond to specific operating conditions.
In order to minimize engine performance losses due to leakage of pressure to outside the flowpath from between a stationary casing and the rotary outer rotor at a relatively high pressure upstream region of the flowpath through the power turbine, a single labyrinth seal has been used between the casing and rotor to deter passage of air. One typical prior art labyrinth seal is composed of a plurality of seal teeth axially spaced, circumferentially extending and outwardly projecting on an annular shaped rim attached to the rotary rotor and a stepped honeycomb structure attached to the stationary casing which sealably interface with the teeth.
However, the seal is located in a thermal environment where the temperature of the teeth is influenced by the higher temperature rotor in contact with the hot gas flow, whereas the temperature of the honeycomb structure is influenced by the lower temperature casing in contact with outside air. This produces a temperature differential between the seal parts which creates a problem. The inner teeth-bearing rim undergoes faster thermal growth than the outer honeycomb structure. This results in increased rubbing of the teeth against the honeycomb structure which makes seal clearance control more difficult.
One approach to solving this problem is presented in U.S. Pat. No. 4,513,975 to Hauser et al, assigned to the same assignee as the present invention. In the Hauser et al patent, a series of axially spaced conduits are provided which join upstream with downstream regions defined by the steps of the stationary honeycomb structure and the teeth on the rotary rotor. The conduits conduct a portion of hot gas from the upstream regions to the downstream regions. Each conduit includes an annular cavity with pluralities of inlet and outlet passages offset circumferentially from one another. Each inlet passage is aligned along a radius of the engine, whereas each outlet passage is aligned within an axial plane and extends at an acute angle relative to the engine axis.
As the temperature of the gas increases, the gas portion passing through each conduit f rom the upstream region assists the downstream region to heat more rapidly. The objective is to increase the rate of heat transfer between seal members and decrease the time it takes to reach thermal equilibrium.
The approach of the above-cited patent is a step in the right direction toward reducing the problem of differential thermal growth between the seal members and its deleterious effects. However, it is perceived that further improvements are needed in this approach in order to devise a practical solution to the problem.